There are a large number of references relating to the preparation of an abrasion-resistant coating on a solid substrate such as a plastic material. In light of the various advantages of plastic materials such as light weight, low material cost and ease of shaping, the development of abrasion-resistant coatings for plastic materials is highly significant from a commercial standpoint. Included among those methods commonly used for such a purpose are a group of methods where a liquid coating composition is applied to a solid substrate and another group of methods where a coating layer is prepared by use of a plasma polymerization of a monomer onto the surface of a solid substrate. Various liquid coating compositions suitable for the first group of methods and various monomers suitable for plasma polymerization have been disclosed in the past. There also exist references relating to the post-treatment of a coated layer which improves or modifies the surface properties thereof.
Burzynski et al, U.S. Pat. No. 3,451,838 describes a process of coating plastics with an organosiloxane. It discloses that abrasion resistant organpolysiloxane compounds can be prepared by the hydrolysis and condensation of at least one compound embraced by the general formula T.sub.n Si Z.sub.4-n where each T is independently a hydrocarbon radical such as alkyl, alkenyl and aryl and each Z is independently a hydrolyzable group such as halogen, acyloxy and aryloxy.
Krekeler, U.S. Pat. No. 3,713,880 describes a process for coating the surface of transparent thermoplastic resins with a solution of a mixture of alkyl silicate, or organosilane and an antistatic additive and thereafter subjecting the coated material to a heat treatment, said organosilane being a compound of the formula R Si X.sub.3, R.sub.2 Si X.sub.2 or a mixture thereof, where R is a hydrocarbon radical and X is a hydrolyzable group, namely, a halogen or a lower alkoxy group.
Gagnon, U.S. Pat. No. 3,650,808 describes a process for providing an abrasion resistant coating on a polycarbonate surface which comprises priming the polycarbonate surface with a compound of the formula H.sub.2 NR.sub.1 Si(OR).sub.3 where R.sub.1 is an alkylene group, and OR is an alkoxy group of 1-4 carbon atoms, and thereafter applying to the surface a coating liquid which is produced by heating methyltrialkoxysilane or a mixture of methyltrialkoxysilane and phenyltrialkoxysilane followed by a partial condensation thereof.
Clark, U.S. Pat. No. 3,986,997 discloses a pigment-free aqueous coating composition comprising a dispersion of colloidal silica in a lower aliphatic alcohol-water solution of the partial condensate of a silanol of the formula RSi(OH).sub.3 in which R is selected from the group consisting of alkyl radicals of 1 to 3 inclusive carbon atoms, the vinyl radical, the 3,3,3-trifluoropropyl radical, the gamma-glycidoxypropyl radical and the gamma-methacryloxypropyl radical, at least 70 weight percent of the silanol being CH.sub.3 Si(OH).sub.3.
French, U.S. Pat. No. 3,953,115 describes a process for applying an adherent, optically clear, abrasion resistant coating to plastic ophthalmic substrates which comprises (a) forming a partially hydrolyzed solution of a vinyltri(loweralkoxy)silane in a water-miscible volatile organic solvent, the silane concentration being 25-75% by weight, (b) applying a thin, uniform coating of the partially hydrolyzed solution to a clean surface of the ophthalmic lens; (c) maintaining the coated substrate in a high humidity and preferably elevated temperature environment until the silane is substantially completely hydrolyzed; and (d) dehydrating (curing) the coated substrate under low humidity conditions at an elevated temperature.
Frye, U.S. Pat. No. 4,277,287 describes an organosilanol coating liquid comprising a dispersion of colloidal silica in an aliphatic alcohol-water solution of the partial condensate of a silanol of the formula RSi(OH).sub.3, wherein R is selected from the group consisting of alkyl having from 1 to 3 carbon atoms and aryl, and a small amount of a polysiloxane polyether copolymer, at least 70 weight percent of the silanol being CH.sub.3 Si(OH).sub.3, said composition containing 10 to 50 weight percent solids.
Kray, U.S. Pat. No. 4,298,655 describes an organosilanol coating liquid comprising a dispersion of colloidal silica in an aliphatic alcohol-water solution of the partial condensate of a silanol of the formula RSi(OH).sub.3, wherein R is selected from the group consisting of alkyl having from 1 to 3 carbon atoms and aryl, a small amount of a betadicarbonyl compound, at least 70 weight percent of the silanol being CH.sub.3 Si(OH).sub.3, said composition containing 10 to 50 weight percent solids.
Suzuki et al, Japan Patent 1839/1980 describes a method of surface treatment of articles wherein the hydrolysis product of a silicon compound having a general formula R.sup.1 C(.dbd.CH.sub.2)C(.dbd.O)OR.sup.2 Si(OR.sup.3).sub.3 (where R.sup.1 is hydrogen or methyl, R.sup.2 is an alkylene group of 1-6 carbon atoms, and R.sup.3 is a hydrocarbon group of 1-8 carbons or an acyl group of 1-4 carbons) is coated on a substrate and thereafter the coating is post-treated with an electron beam having an extremely high kinetic energy, namely 0.3-3 mega electron volts in order to enhance the abrasion resistance of the coating. The inventors state that the hardening of the coating is believed to be due to the polymerization of acrylate or methacrylate groups effected by the irradiation of the very high energy electron beam.
Berger et al, U.S. Pat. No. 4,225,631, describes a process for making an abrasion resistant coating on a polymeric substrate which comprises applying a coating solution of hydrolyzed vinyl(loweralkoxy)silane in a water miscible volatile organic solvent, curing the coated substrate, and subsequently subjecting the cured substrate to ultraviolet radiation. The Berger method relies on the ultraviolet-induced cross-linking of the vinyl groups to produce a harder coating and a better chemical adhesion between the coating and the substrate. Thus, it is an essential requirement in the Berger method that the main ingredient of the starting coating composition be a trifunctional silane monomer having a vinyl group attached to the silicon atom. The term "trifunctional" as used herein signifies that the central silicon atom has three groups attached thereto which are hydroxy groups or groups hydrolyzable to hydroxy groups such as alkoxy groups or halogens. People skilled in the art also use terms tetrafunctional, difunctional and monofunctional silane monomer. They have the corresponding meanings. For a more detailed explanation of said terms as well as the chemistry of silicon, particularly the chemistry of siloxanes, the reader is referred to standard treatises such as Rochow, "An Introduction to the Chemistry of Silicon", 2nd Ed., John Wiley, New York (1951).
When all of the three OH groups of an organotrisilanol RSi(OH).sub.3 undergo condensation with OH groups attached to other silanol molecules, the trifunctional silanol gives rise to a trifunctional structural unit (T unit) of polysiloxane network. Said T unit can be depicted by the formula ##STR1## Similarly, a difunctional silanol of the formula R.sub.2 Si(OH).sub.2 gives rise to a difunctional structural unit of polysiloxane network, and the tetrafunctional silanol Si(OH).sub.4 gives rise to a tetrafunctional structural unit of polysiloxane network. Said difunctional structural unit (D unit) and tetrafunctional structural unit (Q unit) can be depicted respectively by the formulas ##STR2## In the above three formulas, each unit includes half of each associated oxygen atom. Needless to say, difunctional silanol gives rise primarily to a linear siloxane chain, whereas trifunctional silanol gives rise to a cross-linked siloxane network.
It is well known in the art that organosiloxane hard coatings are prepared generally from organosilanol liquid compositions whose main ingredient is trifunctional organosilanol or a partial condensate thereof, whereas softer materials such as siloxane release agents are prepared generally from organosilanol compositions whose main ingredient is difunctional organosilanol.
Quite often, organosilanol molecules are formed by virtue of the in-situ hydrolysis of the corresponding hydroxyzable organosilane molecules, such as alkoxy substituted organosilane molecules. The in-situ formed organosilanol molecules present in the coating liquid usually undergo some degree of condensation before the coating liquid is applied to the substrate. In order to avoid repetition, all three forms shall be deemed equivalent to each other for the purpose of describing this invention. Thus, the term organosilanol as used in the specification and the appended claims shall subsume the precursor and the partial condensation product thereof. Precursors of tetrasilanols are not included within the difunction of an organosilanol since they hydrolyze to a completely inorganic molecule.
Kaplan et al, U.S. Pat. No. 3,843,399 describes a metalized video disc having an insulating layer thereon, where glow discharge is employed to coat the conductive video disc with a polymeric film to obtain a uniform tough dielectric coating which can be repeatedly contacted with a metal tipped stylus without damage. It is stated that monomers suitable for use in forming thin coatings on video discs by glow discharge polymerization include styrene; substituted styrenes; alkyl-substituted silanes such as triethylsilane, trimethylsilane; tetraethysilane, vinyltrimethylsilane and the like; alkenes and cycloalkenes; alkene-substituted benzenes such as divinylbenzene and the like; halogenated compounds such as tetrafluoroethylene, methylene chloride and the like; and polysiloxanes such as dimethylpolysiloxane and the like.
Mehalso, U.S. Pat. No. 4,018,945 describes a method of improving the long term durability of a dielectric polymer film deposited on a video disc by glow discharge polymerization of a dielectric polymer precursor such as styrene, wherein the dielectric polymer is post-treated by a glow discharge in the presence of an oxygen containing gas.
Mehalso et al, U.S. Pat. No. 3,901,994 describes a metalized disc having a dielectric coating thereon wherein a poly-p-xylylene coating is deposited on the metalized disc by a technique such as vapor depostion and then hardened by exposure to a glow discharge.
Kaganowicz et al, U.S. Pat. No. 4,072,985 describes a video disc having a dielectric layer formed from styrene in a nitrogen atmosphere in a glow discharge. It is stated that the dielectric layer has improved age deterioration resistance, wear characteristics and adhesion to a metal conductive layer. This is an example of plasma polymerization.
Nowlin et al, U.S. Pat. No. 4,123,308 describes a process for chemically bonding a poly-p-xylylene to a thermosetting resin, wherein a low temperature plasma is employed to chemically modify the surface of the poly-p-xylylene to incorporate oxygen atoms into the backbone of the polymer at its surface.
Wydeven et al, U.S. Pat. No. 4,137,365 describes an oxygen plasma post-treatment of plastic surfaces coated with plasma polymerized silane monomer wherein a plastic surface is first coated with a polymerized organosilane by use of a plasma polymerization technique conducted in vapor phase and then the coated material is post-treated with an oxygen plasma. It is stated that such oxygen plasma treatment of the coating improves its abrasion resistance.
The Wydeven et al disclosure is directed to a situation where a coating is prepared by plasma polymerization of an organosilane monomer having a sufficient vapor pressure to conduct the polymerization in the vapor phase. As examples of organosilanes suitable for their invention, there are mentioned in the patent vinyltrichlorosilane, tetraethoxysilane, vinyltriethoxysilane, hexamethyldisilane, tetramethylsilane, vinyldimethylethoxysilane, vinyltrimethoxysilane, tetravinylsilane, vinyltriacetoxysilane, and methyltrimethoxysilane. Wydeven et al show that the oxygen-plasma post-treatment incorporates oxygen atoms to the polymer treated. There is not teaching in the patent as to plasma treatment of a coating obtained from a liquid coating composition comprising an organosilanol compound or compounds such as the composition described in the aforementioned Clark, Frye or Kray patent. In fact, the inventors teach away from using liquid "dip" coating compositions because of problems in controlling the film thickness which in turn affects the abrasion resistance and optical properties of the coated article.
Kubacki, U.S. Pat. No. 4,096,315 describes a process for coating an optical plastic substrate which comprises steps of exposing the substrate to a first plasma that forms hydroxyl groups on said substrate's surface, exposing the substrate to a plasma polymerization using a silicon containing monomer, and exposing the substrate to another plasma treatment in the presence of a gas selected from noble gases, oxygen, nitrogen or air. As examples of suitable silicon containing monomers to be used in the plasma polymerization step, there are mentioned in the patent vinyltrimethylsilane, vinyltrimethylethoxysilane, vinyldimethylethoxysilane and hexamethyldisilane.
In Chapter 4 of a treatise entitled "Thin Film Process", edited by Vossen and Kern, Academic Press, New York (1978), Yasuda comments on the chemistry involved in the plasma polymerization of three types of hydrocarbon molecules, namely, triple-bond-containing and aromatic compounds (Group I), double-bond-containing and cyclic compounds (Group II), and compounds without the aforementioned structures (Group III). The author states that under glow discharge polymerization conditions Group I forms polymers by utilizing the opening of triple bonds or aromatic structures with the least evolution of hydrogen gas, that Group II forms polymers via both the opening of double bonds or cyclic structures and hydrogen abstractions, the production of hydrogen gas being considerably higher than Group I compounds, and the Group III compounds polymerize primarily by hydrogen abstraction, hydrogen production being much higher than in those in Group II compounds. Based on this explanation of the plasma polymerization process, it follows that when a vinyl-group-containing silane monomer such as vinyltrimethoxysilane, vinyltriethoxysilane or vinyldimethylethyoxysilane is plasma polymerized, a substantial amount of carbon-carbon polymer backbone will be formed rather than siloxane type bonds.
Hurst, U.S. Pat. No. 3,632,386 describes an oxidative treatment, e.g., electric discharge (corona) or flame treatment of a silicone polymer release surface prepared from a silicone polymer release agent such as solvent-soluble liquid or solid curable silicone rubber polymers, whereby the release properties of the silicone polymer surface is reduced. It is stated in the patent that usually, the silicone polymer release agents are believed to have the formula: ##STR3## R being a monovalent hydrocarbon radical, thus indicating that the silicon-containing polymer is substantially linear without any appreciable extent of cross-linking. The Hurst patent is not directed to abrasion resistant coatings and nothing is mentioned in the patent about the abrasion resistance of the coated surface.
The aforementioned prior art references can be broadly classified into (1) those relating to organosiloxane-type coatings prepared from an organosilanol-type coating liquid by applying the liquid to a substrate and curing the coat by drying and/or heating, (2) those relating to the plasma polymerization of organosilane monomers, (3) those relating to the plasma polymerization of non-silicon containing organic monomers such as xylene or styrene, (4) those relating to the plasma treatment of certain types of coated substances, namely, plasma polymerized organosilanes, plasma polymerized hydrocarbons such as xylene and styrene, and linear silicone rubber type polymers and (5) post-treatment of carbon-carbon double bonds remaining in the coated material either with UV light or with an electron beam having a kinetic energy of mega volts range. None of the aforementioned references are directed to a glow discharge post-treatment of organosiloxane coating obtained by applying an organosiloxanol coating liquid to a solid substrate in order to improve its abrasion resistance. The term "post" as used herein shall mean subsequent to the curing of the coated material by a conventional means such as drying and/or heating.
The present inventors have discovered that when an organosiloxane type coating prepared from a common organosilanol coating liquid comprising trifunctional organosilanol molecules of the formula RSi(OH).sub.3, R being a hydrocarbon radical, or a partial condensation product thereof is post-treated by a glow discharge by placing the coated substrate preferrably in the positive column region of the glow discharge, the abrasion resistance of the coating layer is enhanced to a surprisingly large value, quite often approaching that of a common glass. This great enhancement of the abrasion resistance beyond those of organosiloxane coatings prepared from such organosiloxane coating liquids by conventional curing methods such as drying and/or heating but without glow discharge post-treatment is truly unexpected.
The organosilanol coating liquid composition used in this invention comprises as a main ingredient organosilanol molecules, at least 90 mole percent of the organosilanol molecules being trifunctional organosilanol molecules of the formula RSi(OH).sub.3 or a partial condensation product thereof, where R is a hydrocarbon radical, and at least 50 mole percent of said trifunctional organosilanols being an alkyltrisilanol having one to three carbon atoms, preferably one or a phenyltrisilanol. More preferably at least 70 mole percent of the trisilanol should be methyltrisilanol. Thus, there may be present a minor amount of other organosilanol molecules such as difunctional organosilanol molecules or a partial condensation product thereof. The coating liquid composition used in this invention may contain other ingredients. A typical example of such other ingredients is colloidal silica or tetrasilanol or its precursor. See for instance the afore-mentioned Clark, Frye and Kray patents. Usually the colloidal silica is dispersed in a lower aliphatic alcohol-water solution of the partial condensate of organosilanol molecules. Suitable examples of such lower aliphatic alcohol includes methanol, ethanol, isopropanol and t-butanol. Isopropanol is particularly preferred. Needless to say, mixtures of such alcohols can be used. Optionally, minor amounts of other water-miscible polar solvents such as acetone may be present in the coating liquid.
The term "organosilanol coating liquid" as used in connection with the instant invention shall mean the coating composition as defined above dispersed or dissolved in a suitable medium. Examples of such coating liquids are disclosed in various references, for instance, Clark U.S. Pat. No. 3,986,997, Frye U.S. Pat. No. 4,227,287 and Kray U.S. Pat. No. 4,298,655. The disclosures of these three references are hereby incorporated by reference. The term "organosilanol coating" as used in connection with the instant invention shall mean an abrasion resistant coating prepared from said organosilanol coating liquid.
In the past it has been very difficult, if not impossible, to obtain plastic articles having abrasion resistance anywhere near the abrasion resistance of common glass. As a point of reference in discussing and evaluating the advantages of this invention the following typical abrasion resistance values as measured by a rubbing pad abrader instrument in terms of the number of cycles of an abrasive motion exerted on the surface of the test piece necessary to bring about an increase of 3% absolute in the haze value of the test piece which is expressed as a percent of scattered light in the light transmitted through the test piece. Three percent haze is the level at which haze in a lens becomes noticeable and objectionable to consumers. Details of the testing method used to evaluate the abrasion resistance of materials are described later in the specification.
TABLE I ______________________________________ ABRASION RESISTANCE OF COMMON MATERIALS Material Cycles for 3% Increase Haze ______________________________________ Polycarbonate 7-11 Acrylic resin 14-19 Allyl diglycol carbonate resin 125-150 (CR-39 resin) Glass 3000-6000 ______________________________________
It has also been discovered in this invention that the glow discharge need not be conducted in a special atmosphere and that any gas or vapor may be used as a glow discharge medium. This has significant commercial advantages when the inventive treatment is applied to the commercial treatment of, for instance, a large number of organosiloxane coated acrylic lenses where a large amount of water vapor is released from the lenses at low pressure and it can be used as a glow discharge medium without any substantial addition of a special gas such as oxygen. The elimination of the need for the addition of an extraneous gas not only saves the material cost but also reduces the manufacturing cost, because such addition of a gas to the glow discharge chamber leads to an increased load for the vacuum pump system for the chamber. The abrasion resistance of coated plastic articles obtained according to this invention is much larger than the abrasion resistance of coated plastic articles obtained by the conventional method involving the application of an organosilanol type coating liquid (but without a flow discharge post-treatment), the abrasion resistance obtained by a method involving plasma polymerization of an organosilane plus plasma post-treatment thereof, or the abrasion resistance obtained by plasma polymerization of a hydrocarbon molecule plus a plasma post-treatment thereof.
Additionally, it has been discovered that the abrasion or scratch resistance of mirrored coatings may be substantially improved by the use of this invention. In the past when a mirrored coating was applied to a plastic substrate, the abrasion resistance of the resultant product was generally lower than the abrasion resistance of the substrate. It has been found that this deterioration of the abrasion resistance is not encountered if the substrate is coated with an organosiloxane coating and treated with a glow discharge prior to the application of a mirror layer to the coated surface.